I'll let you guess who tends to write these complaints. That's right ... men.
Mostly, they're men who can't seem to find a dominant woman to have sex with. They complain that dominant women go for guys who aren't as submissive as they are. They claim high and loud that they would be the "perfect submissive;" that they would do absolutely everything to make dominant women happy.
Except, it's not about a woman in particular. It's about all the dominant women - as in interchangeable bodies that fill the fantasy of "dominant woman." Dominant women get objectified, because all women get objectified. She still only exists as an object, a thing that serves a purpose (fulfill the person's kink). She is not seen as a person with her own interests, goals, or tastes.
That kind of thing also happens at parties, not only online. Male submissive newcomers often come to kink parties hoping to just kneel at the feet of some (read any) dominant woman, who will then proceed to a scene with him. To them, a kink event is basically a porn movie they get to walk into.
Dominant Women Should Date Them Because They're Submissive
Which leads me to another way dominant women face sexism while looking for partners: the Nice Submissive syndrome.
These people have adapted the "nice guy" discourse from a submissive point of view. (Here's an excellent primer for the Nice Guy syndrome if you need a refresher.) They believe dominant women owe them scenes and attention simply because they approached them offering their absolute and undying submission.
Just like nice guys who believe they are better than the jerks other girls date, these submissive men proclaim that their submissiveness is so much more submissive than those other, not-really-submissive guys.
When a dominant woman discusses her difficulties finding someone to date, here he is, claiming that he would be so much better at catering to her every need (and, actually, he really expects her to cater to his every need).
We Cannot Escape Sexism
If the experience of my dominant friends is any indication, dominant women have to live in a world where men see them as interchangeable sex dispensers.
Dominant women are extremely fetishized; in some ways, more so than submissive ones. Their power is taken out of their actual person, is dressed up in black leather boots and corsets, and is packaged for men's consumption. When the time comes for dominant women to find real, satisfying relationships, they have to fight stereotypes that are often much more restrictive than those surrounding female submission. Sadly, being a sexually dominant woman doesn't free you from sexism.